Quantum key distribution (QKD) is a well known technique which offers the possibility of secure distribution/generation of cryptographic keys for use in encryption. QKD relies on fundamental quantum properties and allows two parties, commonly referred to as Alice and Bob, to exchange a value and know that an eavesdropper, usually referred to as Eve, has not learnt much about the value. QKD allows key material to be securely derived by Alice and Bob as needed, which offers significant advantages over other methods of key distribution.
Bennett and Brassard described a QKD protocol in C. H. Bennett and G. Brassard, “Quantum cryptography: ‘Public key distribution and coin tossing’,” IEE Conf. Computers Systems Signal Processing, Bangalore, India 1984 which has become known as the BB84 protocol. This protocol uses the transmission of a suitably encoded series of single photons (a quantum exchange) followed by an open discussion via any conventional communication medium (a key agreement stage) to allow Alice and Bob to derive a shared string of random numbers. As single photons are used in the quantum exchange the only way Eve can gain any information about this exchange is to intercept the single photons sent by Alice and measure the information herself. To avoid detection she should also transmit a photon to Bob which attempts to replicate the original photon she intercepted. Due to the random choice of encoding and the quantum nature of the photons Eve can not guarantee to pass a correctly encoded photon to Bob and this will generate a statistical error which will be spotted by Alice and Bob during their conventional communication.
QKD therefore offers a secure means of distributing new key material which protects against eavesdropping.
QKD can also be applied to optical communication networks. British Telecom patent U.S. Pat. No. 5,768,378 teaches that QKD may also be used to distribute keys between a single sender (Alice) and multiple receivers (Bobs) via a passive optical network. Light sent downstream from the Alice end encounters one or more passive optical network switches which distribute the light between their outputs. In terms of sending single photons for QKD each photon traverses one of the downstream paths at random and ends up at one particular Bob. Each Bob can then agree a separate key with Alice. Subsequent message traffic intended for a particular Bob can then be encrypted with the relevant key and sent over the passive optical network. Although all Bobs receive the message traffic only the user at the relevant Bob has the correct key and can thus decrypt the message.
As described above each endpoint requiring a separate key requires a Bob unit, i.e. a quantum receiver. Quantum receivers capable of detecting accurately single photon signals generally require sophisticated cooled detectors and good quality optics.